A phosphino calix[4]arene bis-frustrated lewis pair

Article abstract of DOI:10.1002/ejic.201301529

A bis-frustrated Lewis pair based on a calix[4]arene skeleton is reported for the first time. Importantly, this system has been shown to activate two molecules of hydrogen, thereby establishing a proof of concept for multisite frustrated Lewis pair (FLP) design and synthesis. The complex [calixarene-(Mes₂PH⁺)₂][BH(C₆F ₅)₃^-]₂ (6) has been characterised by X-ray crystal structure analysis. The synthesis and X-ray structure of a calix[4]arene bis-frustrated Lewis pair is described for the first time. This compound is the first example of a frustrated Lewis pair system capable of doubly activating H₂ as well as the first use of a calixarene in frustrated Lewis pair chemistry.

Full text of DOI:10.1002/ejic.201301529

FULL PAPER
DOI:10.1002/ejic.201301529
A Phosphino Calix[4]arene Bis-Frustrated Lewis Pair
Gareth E. Arnott,*[a] Philip Moquist,[b] Constantin G. Daniliuc,[b]
Gerald Kehr,[b] and Gerhard Erker[b]
Keywords: Calixarenes / Phosphorus / Boron / Hydrogen / Lewis acids / Lewis bases
A bis-frustrated Lewis pair based on a calix[4]arene skeleton
is reported for the first time. Importantly, this system has
been shown to activate two molecules of hydrogen, thereby
establishing a proof of concept for multisite frustrated Lewis
pair (FLP) design and synthesis. The complex [calixarene-
] (6) has been characterised by X-
2
ray crystal structure analysis.
–
(Mes2PH+)2][BH(C6F5)3
Introduction
Results and Discussion
Since bis(diphenylphosphanes) have been employed be-
fore in FLP chemistry (Figure 1), we took this as a suitable
starting point for this study. Thus bis(diphenylphosphanyl)-
calix[4]arene 4[8,9] was synthesised by following a literature
procedure that involved quenching the in situ derived distal
dilithio calix[4]arene[10] with chlorodiphenylphosphane
(Scheme 1). Treating bis(diphenylphosphanyl)calix[4]arene
4 with tris(pentafluorophenyl)borane (1:1 ratio) resulted in
a Lewis acid–base pair adduct as evidenced by the appear-
ance of two signals in the 31P NMR spectrum (C6D6) at δ
= –6.1 and 17.2 ppm, respectively (the 31P NMR of calix[4]-
arene 4 has one signal at δ31P = –6.4 ppm[8b]). Significant
Since 2006, when Stephan and co-workers disclosed the
concept of a frustrated Lewis pair (FLP) being able to acti-
vate hydrogen gas,[1] there has been a veritable explosion of
interest in FLP chemistry, particularly since these systems
represent an unusual class of nonmetallic H2 activators.[2]
FLPs have been shown to undergo a number of organic
reactions such as catalytic reduction of imines, enamines,[3]
silyl enol ethers[4] and alkenes;[5] addition to alkenes;[6] and
reaction with CO2.[7] What have been little studied are bis-
FLP systems in which one molecule contains two (or poss-
ibly more) sites for FLP formation. Of the limited examples
of these reported systems (Figure 1),[4] none have resulted
in bis-FLPs capable of doubly activating hydrogen. One op-
tion we thought of pursuing was to use the calix[4]arenes
as a platform for multisite FLP synthesis. To this end, we
wish to report the first use of a calix[4]arene in FLP chemis-
try and the first bis-FLP capable of doubly activating mo-
lecular hydrogen.
1
line broadening in the H NMR spectrum suggested that
the B(C6F5)3 component was not strongly bound to the
phosphorus Lewis base but rather fluctuated between the
phosphanes. Thus it was thought that this system might still
act as an FLP and activate dihydrogen. However, this was
found to not be the case, because stirring a 1:1 mixture of
bisphosphane 4 and B(C6F5)3 in CD2Cl2 under H2 at
1.5 bar returned identical NMR spectra to that without
using hydrogen gas. This result is distinctly different to that
of the bisphosphane FLP systems in Figure 1 but is likely
due to these latter systems being much more sterically con-
gested environments. The calix[4]arene system could, how-
ever, be thought of as two isolated phosphane moieties con-
Figure 1. Examples of bisphosphane-FLPs from the literature.
[a] Stellenbosch University, Department of Chemistry and Polymer
Science,
Private Bag X1, Matieland 7602, Stellenbosch, South Africa
E-mail: arnott@sun.ac.za
http://academic.sun.ac.za/arnott/index.html
[b] Organisch-Chemisches Institut, Universität Münster,
Corrensstrasse 40, 48149 Münster, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201301529.
Scheme 1. Synthesis of bisphosphanyl calix[4]arenes. Reagents and
conditions: (4) (i) nBuLi (2.2 equiv.), THF, –78 °C, (ii) Ph2PCl
(2.5 equiv.), room temp.; (5) (i) nBuLi (2.5 equiv.), THF, –78 °C, (ii)
Mes2PCl (3 equiv.), room temp.
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nected in relative proximity to each other, thus resulting in lix[4]arene salt 6. The 1H NMR spectrum had a doublet at δ
less steric congestion.
= 8.51 ppm (1JP,H = 485.0 Hz) integrating for one hydrogen
Since the diphenylphosphane was clearly unsuitable, the atom, which, owing to the high degree of symmetry,
sterically more demanding dimesitylphosphane was synthe- equated to two protonated phosphonium atoms in this bis-
sised (Scheme 2). This was easily obtained in the same way FLP system. The borohydride gave a nearly indistinct broad
as 4 by quenching the reaction with either dimesitylphen- signal around δ = 3.69 ppm owing to the partial relaxed
ylphosphinite (Mes2P–OPh) or chlorodimesitylphosphane 1JB,H coupling constant.[12] The 19F NMR spectroscopic
(Mes2P–Cl). The latter gave the best yield of 58%, whereas signals [δ = –134 (ortho), –165 (para) and –167 ppm (meta)]
the former reacted slowly to give only 25% bis-phosphane were also somewhat broadened. Both the 31P and 11B NMR
5 after 24 h reaction time at room temperature. The chemi- spectra revealed one doublet each (Figure 2), thereby con-
cal shifts of δ = 6.07 (d, 2 H) and 6.23 ppm (t, 1 H) for firming the phosphonium and borohydride species, respec-
the symmetry-related calix[4]arene aromatic protons on the tively.
unsubstituted rings are considerably shielded, thus indicat-
A sample of bis-FLP calix[4]arene 6 in CD2Cl2 was al-
ing that calix[4]arene 5 adopts a pinched cone conforma- lowed to slowly evaporate at –20 °C. In time, diffraction-
tion[11] in solution with the two phosphane moieties as far quality crystals were obtained and the structure solution
away from each other as possible. Upon adding two equiva- solved with the hydrogen atoms on the P and B atoms being
lents of B(C6F5)3, no changes in the NMR spectra were freely refined (Figure 3). The structure revealed a pinched
observed, thereby confirming the presence of an FLP sys- cone conformation with the aromatic rings that contain the
tem. This 1:2 mixture of 5/B(C6F5)3 in CD2Cl2 was then P atoms being splayed further apart (C4···C19 distance:
treated with hydrogen gas (1.5 bar) and allowed to stir over- 9.697 Å), whereas the adjacent rings (C12···C26 distance:
night. NMR spectroscopy (Figure 2 and Supporting Infor- 5.007 Å) were considerably closer. This was consistent with
mation) confirmed the formation of a bis-phosphonium ca- the solution structure predicted by the upfield resonances
of the aromatic protons in the 1H NMR spectrum [Fig-
ure 2; signals at δ = 6.23 (t, 1 H) and 6.07 ppm (d, 2 H)].
The H–B(C6F5)3 anions were found to be pointing towards
the P–H cations. The P–H bonds are not orthogonal to the
attached calixarene aromatic ring but have torsion angles
of –55.3 (H01–P1–C4–C5) and –43.7° (H02–P2–C19–C20),
respectively. This gives the structure the overall appearance
of a “propeller” with C2 symmetry. Of interest is the close
contact distances for the PH···HB hydrogen atoms; the
H01···H03 contact is 2.01(5) Å and H02···H04 contact
2.15(5) Å. Of the reaction products of boron–phosphorus
FLPs with hydrogen in the Cambridge Structural Database,
Scheme 2. Synthesis of bis(dimesitylphosphanyl)calix[4]arene FLP
and its reaction with dihydrogen. Reagents and conditions: (i)
B(C6F5)3 (2 equiv.), H2 (1.5 bar), CD2Cl2, room temp.
Figure 2. Selected NMR spectra for bisphosphonium calix[4]arene 6 (CD2Cl2).
1395
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the shortest reported distance between hydrogen atoms is but possibly on dendrimers and polymers. Future work will
1.97 Å for [tBu3PH][H–B(2,6-F2–C6H3)3] described re- look at extending the number of phosphine moieties and
cently.[13] Typically the H···H contacts in these systems are the synthesis of “mixed” P/B calixarene systems, as well as
found to be between 2.3 and 3.0 Å.[14] It is noted though the reactivity of these compounds in reduction reactions.
that these H···H contacts are not generally very reliably lo-
cated by X-ray diffraction;[15] Rieger and Repo et al. have
Experimental Section
used neutron diffraction on a hydrogen-activated nitrogen–
boron FLP to show an H···H contact of 1.67 Å[16]
General Procedures: All syntheses that involved air- and moisture-
sensitive compounds were carried out using standard Schlenk-type
glassware (or in a glovebox) under an atmosphere of argon. Sol-
vents were dried with the procedure according to Grubbs[18] or were
distilled from appropriate drying agents and stored under an argon
atmosphere. NMR spectra were recorded with a Bruker AC 200 P
(1H: 200 MHz, 31P: 81 MHz, 11B: 64 MHz), a Bruker AV 300 (1H:
300 MHz, 13C: 76 MHz, 31P: 122 MHz, 11B: 96 MHz, 19F:
282 MHz), a Bruker AV 400 (1H: 400 MHz, 13C: 101 MHz, 31P:
162 MHz), a Varian Inova 500 (1H: 500 MHz, 13C: 126 MHz, 19F:
470 MHz, 11B:160 MHz, 31P: 202 MHz) and a Varian UnityPlus
600 (1H: 600 MHz, 13C: 151 MHz, 19F: 564 MHz, 11B: 192 MHz,
31P: 243 MHz). 1H and 13C NMR spectroscopic chemical shifts (δ)
are given relative to TMS and referenced to the solvent signal. 19F
NMR spectroscopic chemical shifts (δ) are given relative to CFCl3
(external reference), 11B NMR spectroscopic chemical shifts (δ) are
given relative to BF3·Et2O (external reference). 31P NMR spectro-
scopic chemical shifts (δ) are given relative to H3PO4 (85% in D2O)
(external reference). NMR spectroscopic assignments were sup-
ported by additional 2D NMR spectroscopic experiments. Elemen-
tal analyses were performed with an Elementar Vario El III. IR
spectra were recorded with a Varian 3100 FTIR (Excalibur Series).
Melting points were obtained with a DSC 2010 (TA Instruments).
HRMS was recorded with a GTC Waters Micromass (Manchester,
UK). For X-ray crystal structure analysis, data sets were collected
with a Nonius KappaCCD diffractometer. Programs used for data
collection were COLLECT (Nonius B.V., 1998); for data reduction,
Denzo-SMN,[19] for absorption correction, Denzo;[20] for structure
solution SHELXS-97;[21] for structure refinement SHELXL-97;[22]
and for graphics, XP (Bruker AXS, 2000). Thermal ellipsoids are
shown with 30% probability, R values are given for observed reflec-
tions and wR2 values are given for all reflections. Exceptions and
special features for compound 6 included four disordered n-propyl
groups and a disordered dichloromethane molecule that were found
in the asymmetric unit. Several restraints (SADI, SIMU, SAME
and ISOR) were used to improve refinement stability.
(NH···HB contacts are also generally shorter than the
PH···HB versions, which are typically in the range of 1.7–
2.2 Å).[17]
Figure 3. Top: View of the cationic structure of 6 {the two [H–
B(C6F5)3]– counter anions and the hydrogen atoms at carbon
centres were omitted for clarity; thermal ellipsoids are shown with
30% probability}. Bottom: Representation of the X-ray crystal
structure of 6 shows the close P–H···H–B contacts (hydrogen atoms
at carbon centres were omitted for clarity). Selected data (distances
in Å and angles in °): P1–B1 4.250, P2–B2 4.396, H01–H03 2.01(5),
H02–H04 2.15(5), P1–H01 1.35(3), P2–H02 1.37(3), B1–H03
1.17(4), B2–H04 1.20(3); C4–P1–C51 113.3(2), C19–P2–C61
112.2(2), C4–P1–C41 111.4(2), C19–P2–C71 116.6(2), C41–P1–C51
117.5(2), C61–P2–C71 113.8(2), C81–B1–C91 106.0(4), C121–B2–
C111 113.5(4), C81–B1–C101 118.5(4), C131–B2–C111 105.9(4),
C91–B1–C101 114.4(4), C131–B2–C121 115.7(4).
CCDC-946437 (for 6) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Materials: Chlorodimesitylphosphine,[23] B(C6F5)3,[24] dibromo-
calix[4]arene 3[10] and bis(diphenylphosphanyl)calix[4]arene 4[8d]
were prepared according to literature procedures.
5,17-Bis(dimesitylphosphine)-25,26,27,28-tetrapropoxycalix[4]arene
(5): Dibromocalixarene 3 (750 mg, 1.0 mmol) and THF (30 mL)
were added to a dry Schlenk flask (100 mL) equipped with stir bar
and cooled to –78 °C. A solution of nBuLi (1.56 mL, 2.5 mmol,
1.5 m in hexane) was added dropwise and the solution was stirred
for 30 min. A solution of Mes2PCl (915 mg, 3.0 mmol) in THF
(5 mL) was added to the reaction. The solution was warmed to
room temp. and stirred overnight. The solvent was removed under
vacuum and the residue was loaded onto an alumina column (50 g,
activity III). Elution with 1:99–1:9 Et2O in pentane and trituration
with Et2O/MeOH afforded the product (660 mg, 58% yield) as a
Conclusion
In conclusion, we have reported the synthesis and struc-
ture of the first molecule to doubly activate hydrogen
through FLP chemistry. This proof of concept suggests that
other single-molecule multisite FLP systems could be syn-
thesised, not only on calix[4]arenes and related compounds, white solid.
Eur. J. Inorg. Chem. 2014, 1394–1398
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NMR Spectroscopic Data of 5: 1H NMR (500 MHz, 299 K, drogen atoms at P1, P2, B1 and B2 were refined freely; others were
3
4
CD2Cl2): δ = 7.11 (d, JP,H = 7.9 Hz, 2 H, 2-HAr), 6.86 (d, JP,H
=
calculated and refined as riding atoms.
2.4 Hz, 4 H, m-Mes), 6.23 (t, 3JH,H = 7.6 Hz, 1 H, 4Ј-HAr), 6.07 (d,
3JH,H = 7.6 Hz, 2 H, 3Ј-HAr), 4.39 (d, 2JH,H = 13.2 Hz, 2 H, 5-Hax),
Supporting Information (see footnote on the first page of this arti-
cle): Full experimental details and NMR spectra for all com-
pounds.
3
4.04 (m, 2 H, OCH2), 3.61 (t, JH,H = 6.7 Hz, 2 H, OCH2Ј), 3.03
2
(d, JHH = 13.2 Hz, 2 H, 5-Heq), 2.28 (s, 6 H, p-CH3Mes), 2.16 (s,
12 H, o-CH3Mes), 1.95 (m, 2 H, CH2), 1.85 (m, 2 H, CH2Ј), 1.07 (t,
3JH,H = 7.4 Hz, 3 H, CH3Ј), 0.91 (t, JH,H = 7.5 Hz, 3 H, CH3)
3
Acknowledgments
ppm. 13C{1H} NMR (126 MHz, 299 K, CD2Cl2): δ = 158.8 (CAr4),
155.6 (CAr1Ј), 142.9 (d, JP,C = 15.6 Hz, o-Mes), 138.2 (p-Mes),
2
137.5 (d, JP,C = 8.0 Hz, CAr3), 135.1 (d, JP,C = 23.7 Hz, CAr2),
3
2
Financial support from Sasol is gratefully acknowledged by G. A.
133.4 (CAr2Ј), 131.8 (d, JP,C = 19.8 Hz, i-Mes), 130.2 (d, JP,C
=
1
3
3.2 Hz, m-Mes), 129.4 (d, JP,C = 9.6 Hz, CAr1), 127.9 (CAr3Ј),
122.1 (CAr4Ј), 77.4 (OCH2Ј), 76.9 (OCH2), 31.3 (C7), 23.9 (CH2Ј),
1
[1] G. C. Welch, R. R. San Juan, J. D. Masuda, D. W. Stephan,
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9056–9060.
3
23.4 (CH2), 23.2 (d, JP,C = 16.0 Hz, o-CH3Mes), 21.0 (p-CH3Mes),
11.1 (CH3Ј), 10.1 (CH3) ppm. 31P{1H} NMR (202 MHz, 299 K,
CD2Cl2): δ = –22.4 (ν1/2 ≈ 1 Hz) ppm.
5,17-Bis(dimesitylphosphonium)-25,26,27,28-tetrapropoxycalix[4]-
arene Bis[hydrido-tris(pentafluorophenyl)borate] (6): Dimesityl-
phosphane calixarene 5 (56 mg, 0.05 mmol), B(C6F5)3 (52 mg,
0.1 mmol) and CD2Cl2 (1 mL) were added to a dry Schlenk flask
(10 mL) equipped with stir bar. The flask was evacuated and re-
filled with H2 (1.5 bar). The solution was stirred overnight and ana-
lyzed by NMR spectroscopy. The solvent was later removed and
washed with pentane to afford product 6 (75 mg, 71% yield) as a
white solid.
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J. Gloede, S. Ozegowski, A. Koeckritz, I. Keitel, Phosphorus
Sulfur Silicon Relat. Elem. 1997, 131, 141–146; b) C. Wieser-
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mann, W. Vogt, E. Paulus, V. Böhmer, J. Am. Chem. Soc. 1988,
110, 6811–6817; b) A. Arduini, M. Fabbi, M. Mantovani, L.
Mirone, A. Pochini, A. Secchi, R. Ungaro, J. Org. Chem. 1995,
60, 1454–1457.
NMR Spectroscopic Data of 6: 1H NMR (500 MHz, 299 K,
1
3
CD2Cl2): δ = 8.52 (d, JP,H = 485.0 Hz, 1 H, HP), 7.41 (d, JP,H
=
15.1 Hz, 2 H, 2-HAr), 7.16 (d, 4JP,H = 4.7 Hz, 4 H, m-Mes), 6.26 (t,
3JH,H = 7.6 Hz, 1 H, 4Ј-HAr), 5.94 (d, 3JH,H = 7.6 Hz, 2 H, 3Ј-HAr),
2
4.50 (d, JH,H = 13.7 Hz, 2 H, 7-Hax), 4.26 (m, 2 H, OCH2), 3.69
(br., 1 H, BH), 3.63 (t, 3JH,H = 6.7 Hz, 2 H, OCH2Ј), 3.19 (d, 2JH,H
= 13.7 Hz, 2 H, 7-Heq), 2.40 (s, 6 H, p-CH3Mes), 2.26 (s, 12 H, o-
CH3Mes), 1.97 (m, 2 H, CH2), 1.86 (m, 2 H, CH2Ј), 1.08 (t, JH,H
3
3
= 7.4 Hz, 3 H, CH3Ј), 0.94 (t, JH,H = 7.5 Hz, 3 H, CH3) ppm.
13C{1H} NMR (126 MHz, 299 K, CD2Cl2): δ = 165.2 (d, JP,C
=
4
3.4 Hz, CAr4), 155.7 (CAr1Ј), 148.5 (dm, JF,C ≈ 236 Hz, C6F5), 147.6
(d, JP,C = 2.7 Hz, p-Mes), 144.1 (d, 2JP,C = 10.4 Hz, o-Mes), 141.3
4
3
(d, JP,C = 15.0 Hz, CAr3), 138.2 (br. d, JF,C ≈ 243 Hz, p-C6F5),
136.8 (dm, JF,C ≈ 247 Hz, m-C6F5), 134.0 (d, 2JP,C = 13.1 Hz, CAr2),
3
132.6 (d, JP,C = 11.4 Hz, m-Mes), 132.0 (CAr2Ј), 127.9 (CAr3Ј),
125.5 (br., i-C6F5), 122.7 (CAr4Ј), 110.6 (d, JP,C = 86.2 Hz, i-Mes),
108.4 (d, JP,C = 87.1 Hz, CAr1), 77.93 (OCH2), 77.90 (OCH2Ј),
1
1
3
31.2 (C7), 23.8 (CH2Ј), 23.7 (CH2), 22.0 (d, JP, C = 8.8 Hz, o-
CH3Mes), 21.5 (d, 5JP,C = 1.0 Hz, p-CH3Mes), 10.9 (CH3Ј), 9.9 (CH3)
ppm. 11B{1H} NMR (160 MHz, 299 K, CD2Cl2): δ = –25.3 (ν1/2
≈
90 Hz) ppm. 11B NMR (160 MHz, 299 K, CD2Cl2): δ = –25.3 (d,
1JB,H ≈ 85 Hz) ppm. 31P{1H} NMR (202 MHz, 299 K, CD2Cl2): δ
= –16.7 (ν1/2 ≈ 1 Hz) ppm. 31P NMR (202 MHz, 299 K, CD2Cl2):
δ = –16.7 (dm, 1JP,H = 485.0 Hz) ppm. 19F NMR (470 MHz, 299 K,
CD2Cl2): δ = –133.7 (br., 2 F, o-C6F5), –164.6 (br., 1 F, p-C6F5),
–167.5 (br., 2 F, m-C6F5) ppm [Δδ19Fm,p = 2.9 ppm].
[12] B. Wrackmeyer, R. Koster, Analytik der Organobor-Verbin-
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X-ray Crystal Structure Analysis of 6: C112H94B2F30O4P2·CH2Cl2,
Mr = 2242.36, colourless crystal, 0.30 ϫ 0.20 ϫ 0.01 mm, a =
25.4726(11) Å, b = 17.8273(14) Å, c = 23.4402(9) Å, β = 95.451(5)°,
V = 10596.2(10) Å3, ρcalcd. = 1.406 gcm–3, μ = 1.763 mm–1, empiri-
cal absorption correction (0.619ՅTՅ0.982), Z = 4, monoclinic,
space group P21/c (no. 14), λ = 1.54178 Å, T = 223(2) K, ω and φ
scans, 86390 reflections collected (Ϯh, Ϯk, Ϯl), [(sin θ)/λ] =
0.60 Å–1, 18174 independent (Rint = 0.125) and 9323 observed re-
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Received: December 5, 2013
Published Online: January 31, 2014
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